Can I get +12v and -12v from a 9v battery?

[Cliff Schecht]

I would but it requires custom magnetics and you can't just go out and buy a single core to make one (used a toroidal core). You have to buy at least 100 at a time to even have them talk to you. That and I'm probably not supposed to share it .

[armdnrdy]

Gotcha! 

I thought I'd ask. I'm always on the lookout for the "better mouse trap" bipolar supply.

[alparent]

My last attempt at not buying yet another power adapter.

I have a 30vDC 500ma adapter.

Any easy way of getting +15/-15 out of it?

Yes, I will Google this while I'm waiting for you guys to help. 

[bluebunny]

My last attempt at not buying yet another power adapter.

I have a 30vDC 500ma adapter.

Any easy way of getting +15/-15 out of it?

Yes, I will Google this while I'm waiting for you guys to help. 

Voltage divider?  Like we commonly use to make a 4.5V "Vb" from 9V?

[alparent]

Could it be that simple?
I was thinking about it, but I'm not sure enough.

I know I would get +15V and the center and +30V at the top. But if I make the center my GND would I get -15v at the bottom and -15v at the top? Do know if I'm making any sense?
I think I'm missing something?

Don't know if it would be applicable to my need?

I guess I could always try it!  

Don't know what kind of values to use with that kinda voltage?

[R.G.]

Could it be that simple?
I was thinking about it, but I'm not sure enough.

I know I would get +15V and the center and +30V at the top. But if I make the center my GND would I get -15v at the bottom and -15v at the top? Do know if I'm making any sense?
I think I'm missing something?

It is not that simple.

When you make a voltage with a voltage divider, this happens because the two resistances in series let through a current, and that current causes a voltage to appear across each resistor. If the resistors are equal, and the same current flows through them, then the voltages they make by Ohm's law are equal too.

If you pull out or push in any current at the middle point, the resistors no longer get equal currents, so the voltage across them change, and the divider is no longer making the same voltage.

There is a way to figure out what happens. Two resistors in a voltage divider setup make a voltage equal to the unloaded voltage. For instance, if you have 9Vdc, put two 10K resistors in series across it, then you will find at the junction of the resistors 4.5Vdc **as long as there is no loading**. When you load it, the voltage sags. About a page of algebra will show that this voltage acts the same as a "perfect" voltage source of that same 4.5V (or whatever the resistor values tell it to be) running through a series resistor that is numerically equal to the two voltage divider resistors in parallel.

In the example, 9Vdc through a voltage divider of two 10K resistors acts like a perfect 4.5V voltage source running through a single 5K resistor. So if you load this with a 100uA load, the 4.5V sags by 100uA*5K = 0.5V. And you can never pull more than 4.5V/5K = 900uA through it, as this is the current that flows when the output is shorted.

In your example of getting +/-15V by a voltage divider across 30Vdc floating, yes, you can do that. But the current capability of currents flowing into or out of that voltage divider is limited by the resistances in the voltage divider.

Worse yet, if you have to make the current in/out of the divider more than a few milliamperes, the current THROUGH the two voltage divider. This is purely wasted as heat. In the example, we make 4.5V with two 10K resistors. There is 450uA of current flowing through those resistors. And the divider is impractical with load currents much over 50uA. As a general rule, you have to waste ten times as much current in a voltage divider as you can pull out without grossly offsetting it.

What you *can* do is make a voltage splitter. This uses a voltage divider, and a big, burly (relative to anything else in the circuit) voltage follower to make the center voltage be low impedance. Something like an opamp for low power stuff, an audio power amp for bigger currents, with its + input tied to that low-current voltage divider.

But this gets complicated fast.

[armdnrdy]

30vdc which is not a very efficient way of producing 12 volts as we've seen. If you are intent on using the 30 volt transformer look to a MC34063A.

This DC converter can accept an input voltage up to 40 volts, and can produce up to 1.5 amps. It can be set up as an inverter for the negative voltage. There are online calculators for the external components which makes design a bit easier. You'll need to heat sink the regulators because your dropping quite a ways from 30 volts.

One more thing....The MC34063A costs next to nothing! That's always a plus. 

[alparent]

What you *can* do is make a voltage splitter. This uses a voltage divider, and a big, burly (relative to anything else in the circuit) voltage follower to make the center voltage be low impedance. Something like an opamp for low power stuff, an audio power amp for bigger currents, with its + input tied to that low-current voltage divider.

But this gets complicated fast.

Something like this?
http://powersupply88.com/unipolar-to-bipolar-dc-converter-30v-to-%C2%B115v.html

also looking up the MC34063A

[R.G.]

Something like this?
http://powersupply88.com/unipolar-to-bipolar-dc-converter-30v-to-%C2%B115v.html

Yes, but I'd use something like the LM1875 or TDA2030 power amplifier chip.

And this gets the voltages right, but the wirning to keep ground quiet is still tricky.

[Jdansti]

Wouldn't this work if he used his 30VDC 0.5A PS?  Sure the regulators are going to be toasty even with heat sinks, but they're designed to handle it.  It seems to me that this is the simplest option.  I've used a very similar circuit to power a noise gate with no problems.  He's using a wall wart, so if he loses some power to heat, it's no problem as long as the regulator circuit and effect together draw less 0.5A.

[Mark Hammer]

Glad somebody pointed out that these synth type designs really do require a rigid power supply design or things will start to drift all over the place. Even if you implement something fancy like a flyback, you really have to overdesign the "unregulated" negative output so it doesn't droop under heavy loading.

I like linear regulators to drive my synth power supply circuits (even if the linears are powered by a switching supply) because it's simple to obtain good PSU accuracy and regulation on both the positive and negative rails. I designed a switcher for PAiA that output +/-15V which we used to drive +/-12V regulators. Worked really well IMHO.

Well, you're right, but less so in this context.  Were these projects something that required the sort of stability and tracking demanded of melody-producing devices intended to be in tune with something else, you would be spot on.  These projects are self-contained noise/effect producing devices that might be linked to something else, but Alain is just trying to make something self-contained with and for his young son.  So, in this instance, match to what the schematic asks for is more critical than things like stability.

Personally, Alain, I say snag yourself a 9VAC wallwart, and get yourself +/-12-point-something volts out of it with a couple of diodes and some caps.  You can stick all of the 9VAC-to-+/-12v stuff in the chassis, and install a 2.1mm power jack to plug the wallwart into.  It is quite possible I have spares at home that I can bring in to work for you.  That, and a bunch of 1000uf/35v caps I bought dirt cheap several years ago.  Here is an example of how it's done: http://www.paia.com/prodimages/siabsch.pdf

[Cliff Schecht]

You can also use a 1:1 coupled inductor to turn a boost or buck supply into a dual rail supply. One side of the coupled inductor acts as the L in the SMPS and the secondary winding gets one side to ground and a diode/cap for developing DC. I believe there is an example of this in the MC34063 datasheet too.

[amptramp]

Wouldn't this work if he used his 30VDC 0.5A PS?  Sure the regulators are going to be toasty even with heat sinks, but they're designed to handle it.  It seems to me that this is the simplest option.  I've used a very similar circuit to power a noise gate with no problems.  He's using a wall wart, so if he loses some power to heat, it's no problem as long as the regulator circuit and effect together draw less 0.5A.

This is not going to work.  The negative regulator needs a negative supply.  You could use a transformer with a centre tap and a bridge rectifier to generate ± supplies and regulate from there.  Or use two wall warts and connect positive of one to the negative of the other.

[armdnrdy]

I say snag yourself a 9VAC wallwart, and get yourself +/-12-point-something volts out of it with a couple of diodes and some caps.  You can stick all of the 9VAC-to-+/-12v stuff in the chassis, and install a 2.1mm power jack to plug the wallwart into.  It is quite possible I have spares at home that I can bring in to work for you.  That, and a bunch of 1000uf/35v caps I bought dirt cheap several years ago.  Here is an example of how it's done: http://www.paia.com/prodimages/siabsch.pdf

I've been eyeballing this way of producing bipolar voltage but have never implemented it in a build. I'm a bit leery of what I've read about ripple.....but properly regulated and filtered it seems like it should work for what we're doing! Let's throw in proper grounding as well!

Does anyone have first hand experience powering a flanger or phaser using this technique? Any issues?

[Jdansti]

Accidental post- Ignore.   

[armdnrdy]

Accidental post- Ignore.   

I suppose that the cat on your head is to blame! 

[Jdansti]

Yes, and it's also to blame for the incorrect schematic posted above that I can't remove. I can't modify the post anymore, and I've deleted it from photobucket, yet it still lives!  I didn't think the site stored images. Maybe Photobucket just has a lag. I'll keep trying. I'll also take a look at the bipolar PS's I've built and see what I did.

Thanks for the review and comments, Amptramp!



Edit:  

The incorrect drawing is gone, but in the mean time, Mr. Armdnrdy was kind enough to provide me with a replacement photo:



I think it's really special how we help each other out...  

BTW-The unworkable drawing was a poor  attempt to adapt this bipolar supply to a DC input:
http://www.generalguitargadgets.com/pdf/ggg_bipolar_ps.pdf

The transformer in the ggg link above could be an AC wall wart.

[amptramp]

The ggg design would work, but keep in mind that it uses halfwave rectification for each polarity.  I have a ±12 volt wall wart and it is easy to stack two wall warts in series that are fullwave rectified.  If the current drain is light, halfwave is fine.  If not, fullwave works better.  Keeping the ground clean is the hard part.

[PRR]

> deleted it from photobucket, yet it still lives!

I don't see it. You do, because it lingers in _your_ browser cache. Go to another browser, never seen it, and you won't see it neither. Or try Ctrl-F5, forced refresh, that may cause the browser to realize it aint there no more.

And I bet it _IS_ still in PhotoBucket, somewhere; just not via that URL anymore.

[Jdansti]

^ Thanks. It finally disappeared from the forum as viewed on my computational device.